It is often desirable to determine the thermal properties of a medium through which fluid may flow, such as is the case for blood flowing in body tissue, for example. Such properties include the thermal conductivity and the perfusion, i.e., blood flow, thereof. Two exemplary systems for such purpose have been described in U.S. Pat. No. 4,059,982, issued to H. F. Bowman on Nov. 29, 1977 and U.S. Pat. No. 4,852,027, of H. F. Bowman et al., issued on July 25, 1989.
In such systems a thermal transducer probe is inserted into, or positioned adjacent to, a medium whose properties are to be determined. The probe is normally in the form of an approximately spherical thermistor bead which is used both as a heating element and as a temperature sensing element. By passing a small current through the thermistor, temperature is passively sensed by the thermistor to establish a baseline temperature. During a subsequent heating mode a relatively larger current is passed through the thermistor to produce a resistive, or ohmic, heating thereof which is sufficient to maintain the mean volumetric temperature of the transducer at a specified incremental level above the baseline temperature. Such heating operation transfers heat energy to the medium whose thermal characteristics are to be determined. Measurements of the electrical power required to heat the thermal transducer probe and, thereby, the medium to the higher temperature level can then be made and used in a suitably programmed data processor to compute the thermal conductivity and the perfusion characteristics of the medium, as described in the aforesaid Bowman et al. patents.
The thermal transducer probe used in such systems presents a characteristic non-linear response of the heating power to the thermal conductivity k.sub.m of the medium, for example, and normally exhibits relatively low response sensitivity. Typically the non-linear power vs. conductivity response curves of exemplary devices are such that the inverse of the probe heating power is proportional to the inverse of the thermal conductivity of the medium. The general slope of the overall curve over its useful range tends to be relatively low, thereby indicating a generally low response sensitivity. Moreover, the probe displays a "saturation" effect which is such as to produce a further decrease in measurement sensitivity as the medium thermal conductivity increases, i.e., in effect the slope of the curve becomes relatively lower with an increase in thermal conductivity. Similarly, a non-linear and a low sensitivity response of heating power to perfusion also occurs.
The basis for the low sensitivity of such a probe can be understood in terms of effect of the temperature at the interfacial surface of the probe with the medium. As the medium thermal conductivity increases, the temperature at the thermistor/medium interface decreases since the medium environment surrounding the thermistor probe becomes more efficient in extracting thermal energy from the thermistor probe. Accordingly, as the thermal conductivity of the media increases, the measurable temperature field produced by the probe tends not to extend very far into the medium and transducer heating power becomes to a greater extent determined by transducer, rather than medium, thermal properties. Hence, the transducer's ability to thermally "interrogate" the medium becomes less effective, resulting in a thermal saturation effect.
Such undesired non-linearity and lower sensitivity response effects in previously used thermistor probes is a direct consequence of the transducer design, such probes being designed to regulate the transducer mean volumetric temperature rather than the temperature field within the medium whose thermal properties are under investigation. It is desirable to design a thermal transducer probe which provides effectively linear responses to heater power and which does not produce a reduced measurement response sensitivity as the thermal conductivity or perfusion of the medium increases. With such a probe it should be possible to simplify the computations necessary to determine the thermal properties of the medium under investigation and to provide a relatively high response sensitivity.